Method for increased location receiver sensitivity

ABSTRACT

A first image ( 200 ) of a transmitted signal is detected within a received signal containing a plurality of images of the transmitted signal. This is accomplished by detecting a second image ( 210 ) of the transmitted signal. A set of characteristics of the second image of the transmitted signal is determined, and subsequently used to detect the first image ( 200 ) of the transmitted signal.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a method for improvingsensitivity of a receiver to a specific image of a signal.

BACKGROUND OF THE INVENTION

[0002] In communications systems, multipath is a well-understoodphenomenon. Signals propagate from a transmitter to varying degrees inall directions. While one image of the signal (known in the art as thedirect-path image) travels in a straight line from the transmitter tothe receiver, other images are reflected from any number of surfaces andarrive at the receiver at other times, those times being largelydependent on the length of the path traveled from the transmitter to thereceiver.

[0003] Aside from well-understood issues of fading and destructiveinterference, multi-path propagation is generally welcomed in thecommunications arena. In most cases, the direct-path image mustpenetrate several obstacles before arriving at the receiver. Given alarge number of images of the signal following other paths between thetransmitter and receiver, it is likely that many of them will penetratefewer obstacles and experience less attenuation than the direct-pathimage. Since all images of the signal carry the same information, it isunimportant which particular image is detected and decoded. Thus, thearrival of stronger images that arrive at the receiver by other paths isgenerally beneficial.

[0004] Unfortunately, however, the receiver receiving multipath imagesof a signal from various paths is not beneficial for location systems.While signals in location systems generally carry some informationmodulated onto them by the transmitter, the primary purpose of detectionof these signals is to gain physical information that indicates thespatial relationship between the receiver and the transmitter. Signalstrength or received signal strength information (“RSSI”) systemsestimate the distance from a transmitter to a receiver based on thestrength of the arriving signal, assuming that the signal is attenuatedroughly 6 db each time the distance doubles. Angle-of-arrival (“AOA”)systems estimate the direction of the transmitter from the receiverbased on the angle from which the detected signal arrived. Timing-basedsystems, such as ranging or time difference of arrival (“TDOA”) systems,estimate distances in a system based on the arrival time of signals atreceivers, using the approximation that the signals propagate at aconstant speed. In all of these cases, detection of an image of thesignal other than the direct path image is disruptive, since these otherimages will generally have different amplitudes, come from differentdirections, and arrive at different times than the direct-path image.

[0005] Because of these considerations, a location system must becapable of detecting and characterizing the direct-path image despiteinterference from other images of the same transmitted signal. Rejectingimages other than the direct path image cannot be accomplished by simplefiltering or averaging, since the interfering image is, unlike noise orother emissions, highly correlated with the desired image. For thisreason, many location systems use broadband spread-spectrum techniques.Two images of a signal appear as distinct entities if their arrivaltimes differ by a factor on the order of the inverse of the occupiedbandwidth. It is generally valid to assume that the image arrivingearliest is the direct-path image, since the overwhelming majority ofthe propagation of any signal is through air on the Earth's surface, andthe propagation speed of the signal is substantially equal to the speedof light in free space.

[0006] While using broadband modulation addresses the issue ofdistinguishing the direct-path image from other images arriving alongsubstantially different paths, it assumes that the direct-path signal isstrong enough to be detectable. As mentioned previously, the direct-pathsignal is usually not the strongest, as it must generally penetratemultiple barriers, such as walls, floors, vehicles, or buildings beforereaching the receiver. This issue is compounded by the fact that, whilea single connection between a transmitter and a receiver is adequate forcommunication, most location systems require characterization of thepropagation paths between the transmitter and several receivers,increasing the required system density significantly. It is well knownin the art that, by significantly reducing the information capacity of asystem relative to its bandwidth (e.g., by sending largely redundantinformation), it is possible to increase the effective gain of thesystem. This increase is called coding gain. Direct sequence spreadspectrum (“DSSS”) is one implementation of this technique, and is usedextensively by modem broadband location systems, including the GlobalPositioning System, (“GPS”). A pseudonoise (PN) code is superimposed onthe data, generally at a higher modulation rate than the data itself.The receiver is given a priori knowledge of the PN code. The rate atwhich symbols are modulated onto a carrier is called the chipping rate,and determines the bandwidth of the signal. The rate at whichinformation is transmitted is called the bit rate, and determines theutilized bandwidth. The ratio is the number of chips per bit, andestablishes the coding gain for a particular bandwidth. The coding gainmay be approximated as 3 db*log₂(N), where N is the number of chips perbit. Coherent averaging, which takes into account signal phase as wellas amplitude, is possible over the number of chips in a bit because thereceiver knows the pattern of the chips within a bit a priori. Becausethe receiver must know this pattern, it cannot contain any information;in information theory, information is by definition unknown to thereceiver, and any data in a message (such as the PN code) that is knownto the receiver before the message is detected is not consideredinformation. This is why the tradeoff between coding gain andinformation rate becomes possible.

[0007] The teaching of prior art communication systems design is thatcoherent averaging to improve the detection of a digital signal can beperformed only over a length of time corresponding to the duration ofone information symbol or less. A symbol is a quantum of information,which corresponds to one bit in the binary system. The reason for thisis that the symbol carries information and, as such, its state cannot bedetermined a priori by a receiver. If the receiver were to have a prioriknowledge of the state of a symbol, information theory holds that noinformation would be contained therein.

[0008] This technique, however, is not without its disadvantages. Mostlocation systems send at least some data with location transmissions.Examples of data needed to supplement location transmissions includestransmitter identification, ephemeris or status data, alerts,configuration data, and control information. Given a particular amountof data to be sent in a location transmission, for each 3 db increase inbandwidth, the duration of the transmission must be doubled. Thisreduces system capacity and battery life proportionally. Someimplementations add a known synchronization pattern to the message inaddition to the transmitted data to improve the detection sensitivity.This could be regarded as the sending of two distinct signals, one forlocation characterization and one for data. It is obvious that this alsoincreases power consumption and decreases system capacity.

[0009] Thus, there exists a need for a technique that is capable ofutilizing the entirety of the transmitted signal to improve thesensitivity of the receiver to the direct-path image of the signal,without imposing excessive limitations on the amount of information thatmay be sent with the message.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] A preferred embodiment of the invention is now described, by wayof example only, with reference to the accompanying drawings in which:

[0011]FIG. 1 illustrates the geometric basis of a network-basedimplementation of the present invention as implemented by two networkdevices belonging to a location system;

[0012]FIG. 2 illustrates the details of the timing of the signalstransmitted and detected by the devices of FIG. 1;

[0013]FIG. 3 illustrates an elementary implementation of the presentinvention, wherein the detected waveform is assumed to be representativeof the transmitted signal;

[0014]FIG. 4 illustrates a more sophisticated form of the presentinvention, wherein an image of the signal is demodulated, and thedemodulated representation of the image is used to represent thetransmitted signal;

[0015]FIG. 5 illustrates a further refinement of the implementation ofFIG. 4, wherein unintentional modulation of the transmitted signal isapplied to the re-modulation process in order to improve therepresentation of the transmitted signal; and

[0016]FIG. 6 illustrates a refinement similar to that of FIG. 5, whereinthe unintentional modulation of the transmitted signal is applied as acorrection to the received signal rather than as a modification to there-modulation of the detected signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] For simplicity, the following discussion of the present inventionassumes that the target device to be located is a transmitter, and thenetwork devices, at known locations, are receivers. Such a configurationis practical in systems employing RSSI, AOA, TDOA, ranging, and othertechniques known in the art. It will be clear to those skilled in theart that the present invention applies generically to any receiver, andis equally applicable to a system, such as GPS, wherein the targetdevice is a receiver and the network devices are transmitters.

[0018] Referring to FIG. 1, a common scenario is depicted in which afirst image of a signal (referred to as the direct-path image, becauseit is following the most direct path between the transmitter andreceiver) traveling a first path 100 from a transmitter 150 to areceiver 160 is blocked by an attenuating obstacle 170. The first path100, or direct path, is by definition the shortest propagation routebetween the transmitter 150 and the receiver 160, and may beapproximated as a straight-line segment between the transmitter 150 andthe receiver 160. Simultaneously, a second image of the same signaltravels along a second path 110 to a reflective object 180, and afterreflecting from the object 180, travels along a third path 120 to thereceiver 160. Images of the signal, such as this second image, arereferred to as secondary images; the term “secondary image” encompassesall images other than the direct-path image. Because the secondary imagedoes not penetrate any attenuating obstacles that absorb or reflect itspower, and the direct-path image passes through the attenuating obstacle170, the amplitude of the secondary image at the receiver 160 is largerthan the amplitude of the direct-path image at the receiver 160. Becausethe sum of the lengths of the second path 110 and the third path 120traveled by the secondary image is greater than the length of the path100 traveled by the direct-path image, the secondary image arrives atthe receiver 160 at a later time than the direct-path image.

[0019] Referring to FIG. 2, the direct path image 200 of the signaltransmitted by the transmitter 150 and having traveled along the firstpath 100 to the receiver 160 is shown along with a secondary image 210having traveled from the transmitter 150 to the receiver 160 along thepaths 110, 120 as described above. For simplicity, this discussion willassume that the images 200, 210 are the only significant images arrivingat the receiver 160. Due to the conditions depicted in FIG. 1, thesecondary image 210 arrives at the receiver at a later time and withhigher amplitude than the direct-path signal 200. The arrival time ofthe secondary image 210 differs from that of the direct-path image 200by a delay 230, representing the difference in the length of the pathstraveled by the two signals 200, 210. For simplicity, each image isrepresented by the demodulated data signal that would result from idealdetection of the respective image. Both images are drawn superimposed ona receiver sensitivity limit 220. It can be seen that, while thesecondary image 210 is larger than the receiver sensitivity limit 220,the direct-path image 200 is below the sensitivity limit 220 and wouldnot be detected using the techniques of the prior art. Thus, in theprior art, the secondary image 210 would be identified as the earliestarriving image and would be erroneously assumed to be the direct-pathimage 200, with corresponding detrimental effects on any attempt toestimate the location of the transmitter 150 based on the amplitude,angle of arrival, time of arrival, or any other characteristics of thesecondary image 210.

[0020] The present invention observes that, in a multipath environment,one is dealing with distinct images of a single signal. While onenecessary purpose of this signal may be to convey information from atransmitter 150 to a receiver 160, a location system has the primarypurpose of characterizing physical properties of the direct-path image200 of the transmitted signal, such as arrival time, phase, oramplitude. The sensitivity requirement for detecting a direct-path image200 is generally more difficult than for a secondary image 210 becausethe amplitude of the direct-path image is usually lower than that of themost prominent secondary images, and because secondary images constitutea source of interference with the direct-path image. The presentinvention proposes to dramatically increase the coding gain for thedirect path image 200 by reducing its information content withoutincreasing the length of the transmission. This will offer a coding gainon the order of 3 db*log₂(N), where N is the number of symbols in themessage.

[0021] According to current information theory, an image of a signalcarries no information if the receiver, independent of the detection ofthe image, knows the information modulated onto the signal. The presentinvention proposes that such information about the direct-path image 200of the signal is contained in the secondary image 210 of the signal.Since, in real-world terrestrial location applications, there almostinvariably exist secondary images of the signal that are much larger inamplitude than the direct-path image at the receiver 160, the detectionof at least some of these secondary images 210 may be assumed to begenerally easier than the detection of a direct-path image 200.

[0022] Therefore, the present invention proposes that the receiver 160may detect the secondary image 210 of the signal. The receiver 160 thenhas knowledge of the information content of the secondary image 210 (forexample, the sequence of bits sent by the transmitter 150). Since allother images of the transmitted signal are representative of the sametransmission, the information content of any image of the signal,including the direct-path image 200, is reduced by the amount ofinformation gleaned from the secondary image 210. In practical terms,nearly all of the information modulated onto the transmitted signal canbe determined by the receiver 160 in the process of detecting thesecondary image 200. As no image of the signal, including thedirect-path image 200, has significant information content, coherentaveraging is theoretically possible over the entire length of the imagerather than over a single bit. If there are N bits in the signal sent bythe transmitter 150, using the knowledge of the bits of the secondaryimage 210 to detect the direct-path image 200 enables a coding gain of 3db*log₂(N).

[0023] It will be appreciated by those skilled in the art ofelectromagnetic propagation that this scenario represents anunrealistically simple description of a typical real-world terrestrialpropagation scenario, which would usually include dozens to hundreds ofsecondary images 210 of the transmitted signal, arriving at the receiver160 at various times and with various amplitudes and phases. Thereceived signal at the receiver 160 is the sum of all of these images,along with any other noise and interference. For the purpose of thepresent discussion, it is adequate to assume that the secondary image210 represents an easily detected member of the large set of secondaryimages.

[0024] There are a variety of approaches to modeling the originalsignal. FIG. 3 illustrates a typical receiver structure in which theradio frequency (“RF”) section 300 converts the received signal to areceived waveform 310, represented in either analog or digitized format.This step may optionally include such operations as frequencytranslation, signal conditioning, and filtering. The waveform of thetransmitted signal is one of the most basic characteristics. Thereceived waveform may be represented in any appropriate analog ordigital format, with any resolution required for the application. Thewaveform 310 may be subjected to additional signal conditioning 320,which may include, but is not limited to, filtering, limiting,companding, or other operations appropriate to the modulation used. Theresult of this optional additional processing is the comparison waveform330. If no additional processing is applied, the comparison waveform 330will be identical to the received waveform 310.

[0025] In accordance with the implementation of the present inventionillustrated in FIG. 3, it is assumed that there exists a dominant imageof the signal, such that the comparison waveform 330 substantiallyrepresents the dominant image. This is the case in the two-ray exampledescribed in FIG. 1, and it is a reasonable assumption in manyreal-world cases. The sensitivity improvement effected by the presentinvention is directly related to the degree of similarity between thecomparison waveform 330 and the transmitted signal.

[0026] If the comparison waveform 330 representing the conditionedreceived signal can be assumed to reasonably represent the transmittedsignal, the comparison waveform 330 may then be passed to the correlator360 for comparison with the delayed received waveform 350. In thepreferred embodiment, the correlator 360 compares the conditionedreceived waveform 330 with the delayed received waveform 350 by aconvolution operation, which involves integration of the products ofthose waveforms over a window in time for many values of delay 340. Whenthe delay 340 is equal to the difference 230 in the arrival times of thedirect-path image 200 and the secondary image 210 (plus the delay, ifany, through the signal conditioning block 320), the output 370 of thecorrelator 360 will show a peak. The delay 340 will be varied in orderto find the value(s) at which the correlator output 370 shows a peak.The largest value of the delay 340 at which a significant peak in thecorrelator output 370 occurs is assumed to represent the difference 230in arrival times between the secondary image 210 and the direct-pathimage 200. It is important to perform the operation for values of delays340 in the range of zero to at least the maximum delay spread expectedbetween a direct path image 200 and a significant secondary image 210 inthe system environment, plus the additional delay imposed by the signalconditioning 320. In some circumstances, such as within large mountainranges, the delay spread alone may be in the tens of microseconds, andthe additional conditioning delay is entirely dependent on theimplementation. The delayed received waveform 350 is identical to thereceived waveform 310, but delayed by the desired amount.

[0027] A more sophisticated approach is illustrated in FIG. 4. In thisexample, the received signal 310 is not only optionally conditioned toproduce a conditioned signal 430, but further demodulated by ademodulation block 440 into a digital signal 450 representing thedigital information originally transmitted. This information is the mostimportant characteristic of the transmitted signal from a communicationssystem standpoint. The process of demodulating the signal may include,but is not limited to, detection of the original sequence of symbolssent, translation of the sequence of symbols into a sequence of bits orother information content, recovery of clock, edge, or othersynchronization, error detection, and error correction. If necessary,the digital waveform 450 may be converted back into a modulatedcomparison waveform 330 by a remodulation block 460 for comparison withthe delayed received signal 350. This may not be necessary formodulation techniques, such as binary phase shift keying (“BPSK”), wherethe digital waveform 450 inherently bears a strong similarity to themodulated waveform 330, but will be required for such modulationtechniques as frequency shift keying (“FSK”), minimal shift keying(“MSK”), quadrature phase shift keying (“QPSK”), offset quadrature phaseshift keying (“OQPSK”), quadrature amplitude modulation (“QAM”), orothers where the modulation affects the analog waveform in more indirector abstract ways.

[0028] Again, it must be observed that the sensitivity of the system tothe direct-path signal 200 is directly related to the degree to whichthe comparison waveform 330 represents the original transmitted signal.In many cases, remodulation of the demodulated information mayadequately represent that transmitted signal. However, there are anumber of circumstances where this may be less than optimal. A set ofcharacteristics, referred to as errors, which can be defined asunintentional modulation of the signal, cause the idealized remodulatedwaveform 330 generated from knowledge of the demodulated information todiffer from the actual transmitted signal. When significant modulationerrors are expected in normal system operation, the demodulation block440 will include techniques to detect, characterize, and compensate forthem in the process of recovering the digital signal 450.

[0029] These errors may include, but are not limited to, differences inthe timebases of the transmitter 150 and the receiver 160 (frequencyoffset), unintended variations in the amplitude of the transmittedsignal (envelope variation), jitter or other errors in the boundariesbetween transmitted symbols (symbol skew), inter-symbol interference(ISI), variations in the carrier frequency of the transmitted signalover the course of the message (frequency variation), and errors in theorthogonality of I (in-phase) and Q (quadrature) oscillators in eitherthe transmitter 150, the receiver 160, or both (axis skew).

[0030]FIG. 5 illustrates a system designed to recover those errors andreintroduce them to further improve the resemblance of the comparisonsignal to the original transmitted signal. Error information 550detected by the demodulation block 440 in the process of decoding thedigital signal 450 are passed to the error reproduction block 560, whichapplies them in the remodulation block 460 to the comparison signal 330.By including the reproduced errors in the remodulation process, acomparison signal 330 is generated which bears a closer resemblance tothe transmitted waveform and improves the ability of the correlator 360to detect other images of the signal. This results in an improvement ofsensitivity in direct proportion to the improvement in the similarity ofthe comparison waveform to the transmitted signal.

[0031] In some cases, it is more practical to apply this correction tothe received waveform 310 or the delayed received waveform 350. Forinstance, a frequency offset error between the transmitter and receiverwill result in a constant rate of phase shift observed by thedemodulation block 440. For the purposes of the present invention, thiserror information may be incorporated either by reapplying that phaseshift to the comparison signal 330 to improve its resemblance to thetransmitted signal, or it may be applied as a correction to the delayedreceived waveform 350.

[0032]FIG. 6 illustrates an implementation of the present inventionwhere error data is applied to the delayed received waveform 350 insteadof to the comparison waveform 330. The error information 550 detected bythe demodulation block 440 is passed to the error correction block 660.The error correction block 660 applies a correction to the delayedreceived signal 350, resulting in a corrected delayed received signal650 which is passed to the correlator. The correction will result in theimages of the transmitted signal contained in the delayed receivedsignal 650 bearing a closer resemblance to the ideal remodulated signalrepresented in the comparison signal 330. This will produce acorresponding improvement in the ability of the correlator 360 to detectthe images of the transmitted signal.

[0033] In some cases, the error correction 660 may be applied to thereceived signal 310 before it is delayed by the delay element 340. Inmost cases, however, it will be desirable to apply the correction to thedelayed received signal 350, since the error information 550 from thedemodulator will be valid only when an image has been detected. If theerror correction 660 is applied before the delay element 340, somereal-time architectures may fail to apply the correction to images ofthe transmitted signal arriving earlier than the image decoded by thedemodulation block 440.

[0034] It will be apparent to those skilled in the art of signalprocessing that the implementation of FIG. 5, wherein the detectederrors are used to represent the transmitted signal more accurately inthe comparison signal 330, and the implementation of FIG. 6, wherein thedetected errors are used to correct the delayed received signal,represent variations of the same technique, and are mathematicallyequivalent. An implementation of this invention may use either of thesetechniques, or as easily may combine the two, as appropriate to thesignaling used and the operating conditions involved. It will be furtherapparent that additional improvements may be obtained by detectingadditional images of the transmitted signal and combining thecharacterizations of all detected images to further improve thesensitivity of the system to weaker images, either simultaneously oriteratively.

[0035] While the invention has been described in conjunction withspecific embodiments thereof, additional advantages and modificationswill readily occur to those skilled in the art. The invention, in itsbroader aspects, is therefore not limited to the specific details,representative apparatus, and illustrative examples shown and described.Various alterations, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. Thus, itshould be understood that the invention is not limited by the foregoingdescription, but embraces all such alterations, modifications andvariations in accordance with the spirit and scope of the appendedclaims.

We claim:
 1. A method for detecting a first image of a transmittedsignal within a received signal containing a plurality of images of thetransmitted signal, the method comprising the steps of: detecting asecond image of the transmitted signal; determining a set ofcharacteristics of the second image of the transmitted signal; and usingthe set of characteristics to detect the first image of the transmittedsignal.
 2. The method of claim 1 wherein the first image is adirect-path image and the second image is a secondary image.
 3. Themethod of claim 1 wherein the set of characteristics is at least onecharacteristic of the transmitted signal selected from a groupconsisting of: a sequence of bits, a sequence of symbols, a waveform,phase, amplitude, frequency offset, frequency variation, envelopevariation, symbol skew, axis skew, and inter-symbol interference.
 4. Themethod of claim 1 wherein the first image and the second image aredistinct images of the transmitted signal.
 5. The method of claim 1wherein the step of using the set of characteristics comprises the stepof performing coherent averaging over a length of time to detect thefirst image of the transmitted signal.
 6. The method of claim 1 furthercomprising the step of converting the received signal to a receivedwaveform.
 7. The method of claim 6 wherein the received waveform is usedas a characteristic of the transmitted signal.
 8. The method of claim 6further comprising the steps of: applying signal processing to thereceived waveform to obtain a processed received waveform; and using theprocessed received waveform as a characteristic of the transmittedsignal.
 9. The method of claim 8 wherein the signal processing comprisesat least one of the following techniques: limiting, companding, andfiltering
 10. The method of claim 6 further comprising the step ofapplying an error correction to the received waveform.
 11. The method ofclaim 1 further comprising the steps of: demodulating the second imageof the transmitted signal to determine at least a portion of informationcontent modulated onto the signal image by a transmitter; and using theinformation content as a characteristic of the transmitted signal todetect the first image of the transmitted signal.
 12. The method ofclaim 11 further comprising the steps of: remodulating the informationcontent; and using the remodulated information content as acharacteristic of the transmitted signal.
 13. The method of claim 11further comprising the steps of: characterizing at least one error inthe second image of the transmitted signal; and using at least onecharacterized error as a characteristic of the transmitted signal. 14.The method of claim 1 wherein the second image of the transmitted signalarrives at a receiver at a later time than the first image of thetransmitted signal.
 15. The method of clam 1 wherein the second image ofthe transmitted signal has a higher amplitude than the first image ofthe transmitted signal.
 16. The method of claim 1 wherein the step ofdetermining a set of characteristics comprises the step of decoding atleast a portion of information that is modulated on the transmittedsignal prior to detecting the first image of the transmitted signal. 17.The method of claim 1 further comprising the step of converting thesecond image to a waveform, wherein the waveform is assumed tosubstantially represent a dominant image of the transmitted signal. 18.The method of claim 17 further comprising the steps of: comparing thewaveform with a second waveform that has been delayed; and performing anoperation for values of delays in a range of zero to at least a maximumdelay spread expected between the first image and he second image of thetransmitted signal.
 19. The method of claim 1 wherein informationcontent of the first image is reduced after the set of characteristicsof the second image is determined.
 20. The method of claim 1 wherein acoding gain of the first image is increased after the set ofcharacteristics of the second image is determined.